The present invention is generally directed to an apparatus and method for use in heating, ventilation, and/or air conditioning systems (xe2x80x9cHVAC systemsxe2x80x9d). More particularly, the present invention is directed to an apparatus and method employing a controllable, multipurpose heat pipe system that provides an improved, more energy efficient HVAC system. Advantages of the present invention are especially apparent when the present invention is applied to buildings utilizing centralized HVAC systems.
HVAC systems generally function to heat or cool air to a more comfortable temperature by manipulating the transfer of heat. For example, an air conditioning system may contain a cooling coil that absorbs heat from hot air to lower the air""s temperature. Similarly, a heating system may utilize a heated gas or liquid to transfer heat to cold air to increase the air""s temperature.
Heat transfer from or to air may be effected within such HVAC systems by the use of a working fluid or refrigerant, such as ammonia, R134a (tetrafluoroethane), or similar fluids. These working fluids are generally capable of changing state under various conditions of temperature and pressure. With each change of state, the working fluid either accepts energy or gives up energy. As a result, this energy is either removed or added to the air, respectively, so that cold air may be heated or hot air may be cooled.
In a conventional air conditioning system, the working fluid generally moves in the following cycle of operation: (1) from a compressor; (2) to a condenser; (3) through an expansion valve; (4) to an evaporator; and then (5) back to the compressor. In one such air conditioning system, the working fluid enters the compressor as a low temperature gas at about 65 F. and leaves the compressor as a high temperature gas at about 150 F. The working fluid then enters the condenser. Within the condenser, the working fluid thermally communicates with, and gives up heat to, surrounding cooler air, and the working fluid is converted from a high temperature gas into a cooler liquid of about 90 degrees F. The working fluid then passes through an expansion valve to a region of low pressure. As a result, the working fluid begins to change state from a liquid to a low temperature gas of about 45 degrees F. The working fluid then moves to the evaporator, where the working fluid thermally communicates with, and absorbs heat from, hot air surrounding the evaporator. As heat is transferred from the hot air to the working fluid, the hot air is cooled, and the working fluid is heated to become a gas of about 65 degrees F.
In general, an air conditioning system may provide sensible cooling and latent cooling. Sensible cooling is associated with reducing the temperature of air, while latent cooling is associated with decreasing the moisture content of air (dehumidification). For example, when air is cooled, but is not cooled below its dewpoint, only sensible cooling has occurred. On the other hand, when air is cooled to its dewpoint (approximately 60 degrees F.), moisture in the air begins to condense, and dehumidification (latent cooling) of the air begins. If the air is cooled below its dewpoint, further dehumidification occurs. Therefore, when air is cooled below its dewpoint, both sensible and latent cooling have occurred, because the air""s temperature has been reduced (sensible cooling), and moisture has been removed from the air (latent cooling). In order for conditioned air to be comfortable to humans, the air must be at a comfortable temperature, and it must contain an appropriate level of moisture.
In some air conditioning systems, air is reheated after being cooled and dehumidified. In one such air conditioning system, warm air enters at approximately 80 F. The air then moves through the system""s evaporator and is typically cooled to approximately 55 degrees F. or lower. During this cooling process, when the air temperature reaches its dewpoint (approximately 60 degrees F.), moisture in the air condenses (dehumidification begins). As the temperature of the air falls below the dewpoint, additional moisture is removed from the air. Such cooling normally produces air that is colder than desired for human comfort. However, this degree of cooling is often required to provide the necessary amount of dehumidification. In essence, the air conditioning system""s evaporator generally overcools the air in order to remove an appropriate amount of moisture. Accordingly, some air conditioning systems reheat the dehumidified air to a more comfortable level. Such systems are very energy inefficient, because excess energy is used in the overcooling (dehumidification) and reheating processes.
Heat pipes are passive devices that may be used to heat or cool air by manipulating the transfer of heat from a heat source to a heat sink, or vice versa. Heat pipes may contain a precise amount of working fluid or refrigerant, such as ammonia. The working fluid is generally contained within the heat pipe and may be cycled continuously through at least two elements: an evaporator and a condenser. Adiabatic section(s) may be included in the heat pipe to allow the working fluid to pass between the evaporator and the condenser without transferring heat to or from the surroundings.
Evaporators and condensers are generally configured to allow heat exchange with the environments that surround them. For example, a heat pipe evaporator may be configured to absorb heat from a heat source, such as hot, unconditioned air that passes around the evaporator. Similarly, a heat pipe condenser may be configured to release heat into a heat sink, such as cool, conditioned air that passes around the condenser. As such, a heat pipe may act as either a heater or a cooler depending on its orientation.
In general the heat pipe""s working fluid enters a heat pipe""s evaporator in a liquid state. The working fluid in the evaporator absorbs heat from a heat source, such as hot, unconditioned air. As a result, the hot air may be cooled, and the working fluid is transformed from a liquid to a vapor. The vaporized fluid then passes from the evaporator, through an adiabatic section (in some embodiments), into a condenser. In the condenser, the vapor releases heat to a heat sink, such as cool, conditioned air. As a result, the cool air may be heated, and the working fluid is transformed from a vapor to a liquid within the condenser. The condensed working fluid is then returned to the evaporator, in a liquid state, by the force of gravity and/or capillary forces through a wick. The return path may be through an adiabatic section or other connecting section.
To improve conditioning performance, some air conditioning systems utilize heat pipe heat exchangers. In these systems, heat pipe technology is used to increase moisture removal capacity and/or to provide passive reheat capability. Such systems are disclosed in U.S. Pat. No. 2,093,725 to Hull; U.S. Pat. No. 4,607,498 to Dinh; U.S. Pat. No. 4,971,139 to Khattar and U.S. Pat. No. 5,695,004 to Beckwith.
Generally, conventional dehumidification heat pipe systems have been used in the following manner. Hot air enters the HVAC system. The heat pipe may be used to precool the hot air before the hot air is cooled by the air conditioning system. In such a system, a heat pipe may be disposed around the air conditioning system""s main cooling coil (or main evaporator). Generally, the heat pipe""s evaporator will be positioned upstream of the main cooling coil, while the heat pipe""s condenser is positioned downstream of the main cooling coil. As such, the hot air passes over the heat pipe""s evaporator and heat may be absorbed from unconditioned hot air by the heat pipe""s evaporator. This precool the air. Then the precooled air passes over the system""s main cooling coil and is cooled further and becomes dehumidified. The heat may then be transferred via the heat pipe""s condenser to cool the dehumidified air that leaves the cooling coil. As a result, the main air conditioning system may cool the precooled air to a lower temperature than would otherwise be possible. As described above, by cooling the air to a lower temperature, more moisture may be removed from the air. The cool, dehumidified air passes over the heat pipe""s condenser and is partially reheated to reheat the air that exits the HVAC system. Because heat is absorbed from the unconditioned hot air (by the heat pipe""s evaporator) and then transferred to the dehumidified cooled air (by the heat pipe""s condenser), the dehumidified cooled air may be reheated before it exits the cooling system. As described above, by reheating the air after it has been dehumidified, the air will be more comfortable to humans.
A significant problem with conventional heat pipe systems relates to their use during the cooling season. Generally, moisture must be removed from hot, unconditioned air during the cooling season in order to make the air comfortable to humans. As described above, in order to remove this moisture, the hot air must be cooled to temperatures below its dewpoint (approximately 60 F.). During regular operation, the hot air will be excessively cooled to remove an appropriate amount of moisture. Then the cooled air will be reheated before it exits the cooling system. However, at certain peak load times during the day, one may want to maximize the sensible heat removing capacity of the cooling system by cooling the air to its lowest possible temperature. In such a case, it would be inefficient and counterproductive to reheat the air before it exits the cooling system. Thus, when heat pipe heat exchangers are used in conjunction with an air conditioning system to precool and reheat conditioned air, it may be desirable to control the heat transfer characteristics of the heat pipe exchanger. For example, on days having significant peak loads, it may be desirable to modify or eliminate the heat pipe""s function of reheating cooled air.
Another problem with conventional heat pipe systems relates to their presence in HVAC systems during the heating season. Heat pipe systems generally perform no beneficial function during the heating season, because cold, unconditioned air generally does not require dehumidification. As such, a heat pipe system imposes an unnecessary resistance to the flow of air through the HVAC system during the heating season. As a result, a fan motor of greater horsepower is needed to maintain proper air flow through the system, and the overall energy efficiency of the HVAC system is reduced due to the increased energy consumption of such a fan. Therefore, the energy efficiency of such a system will vary in proportion to the length of the heating season. When the heating season is longer, the annual energy efficiency of the system will be less.
Accordingly, a need exists for a controllable HVAC system that includes a heat pipe that is functional and energy efficient in various environments. In particular, there is a need for a controllable, multipurpose heat pipe system that can operate efficiently during both the cooling and heating seasons.
A further problem with conventional heat pipe systems relates to their use with central HVAC systems that operate in buildings comprised of different heating or cooling zones. In such buildings, the heating or cooling load of each zone may vary at any given time. In the past, heat pipe systems have been disposed around a central cooling coil to provide a means for controlling the temperature and moisture content of air at the central unit. However, these systems did not provide a means for controlling the temperature or humidity within the individual zones.
As such, a need exists for a heat pipe system that can be applied to a central HVAC system for conditioning individual zones. In particular, a need exists for a heat pipe system that can help efficiently control the temperature and humidity of air within the individual zones.
The present invention recognizes and addresses the foregoing disadvantages, and others, of prior art constructions and methods. Accordingly, a primary object of the present invention is to provide a controllable, multipurpose apparatus for controlling air temperature in HVAC systems.
Another object of the present invention is to efficiently manage the latent cooling capacity and sensible cooling capacity of an air conditioning system during various loads by providing mechanisms for controlling the flow of a secondary working fluid within the apparatus.
A further object of the present invention is to improve the design of heat pipe heat exchangers that are used within HVAC systems. A more particular object of the present invention is to provide a heat pipe system that performs typical dehumidification functions during the cooling season and that provides useful functions during the heating season as well.
An additional object of the present invention is to provide a heat pipe system that improves the efficiency of heat transfer within a heating system.
Another object of the present invention is to improve the overall energy efficiency of an HVAC system by providing an apparatus that minimizes airflow resistance within the system.
Still another object of the present invention is to provide an apparatus for controlling the air temperature of individual zones within a building that is conditioned by a centrally located HVAC system.
Some of these objects, and others, are achieved by providing a passive, energy efficient apparatus that is suitable for controlling air temperature in an air passage of an HVAC system. The apparatus employs a secondary heat transfer unit that preferably includes at least one heat pipe. A secondary fluid contained within the heat pipe functions as the working fluid within the heat pipe.
In a presently preferred embodiment of the present invention, the heat pipe further includes at least two conducting sections. In some embodiments, the first conducting section that thermally communicates with the air that is to be conditioned is an evaporator, and the second conducting section is a condenser. In other embodiments, the heat pipe may contain a third conducting section that also functions as a condenser.
In one embodiment of the present invention, a heat pipe evaporator is positioned upstream from an air conditioning system""s main cooling coil. As such, warm air entering the system thermally communicates with the heat pipe evaporator before the air thermally communicates with the main cooling coil. The heat pipe evaporator absorbs heat from the warm air, and, as a result, the warm air is precooled before it reaches the system""s main cooling coil. The heat absorbed by the heat pipe evaporator is transferred to the working fluid within the evaporator. Consequently, the working fluid exiting the heat pipe""s evaporator is vaporized. The vaporized fluid flows from the heat pipe""s evaporator into the heat pipe""s condenser via an interconnecting vapor line, which, in some embodiments, may comprise an adiabatic section.
In some embodiments of the present invention, the heat pipe condenser thermally communicates with a heat sink, such as a cold air mass. For example, the heat pipe""s condenser may be positioned downstream from the cooling system""s main cooling coil, so that the heat pipe""s condenser thermally communicates with cool air leaving the main cooling coil. Alternatively, the heat pipe""s condenser may be positioned within a duct containing cool return air from a cool zone. In further embodiments, the heat pipe""s condenser may be positioned in a cool zone within the building.
When vaporized working fluid passes through the heat pipe""s condenser, heat is removed from the working fluid and is absorbed by the cold air mass (heat sink). As a result, the working fluid is condensed back to its liquid state, and the cold air mass is warmed. The condensed working fluid that leaves the heat pipe""s condenser flows in liquid form, back to the heat pipe""s evaporator via at least one interconnecting liquid line, which, in some embodiments, may comprise an adiabatic section.
In a presently preferred embodiment of the present invention, one or more mechanisms may be provided within the heat pipe to precisely control the amount of heat transferred by the heat pipe. The mechanism(s) may interrupt or modulate the flow of the working fluid between the components of the heat pipe. For example, flow control valves may be installed between the evaporator and the condenser to interrupt or reduce the flow of working fluid from the evaporator to the condenser, or vice versa. Such valves can thereby prevent or reduce heat transfer by the heat pipe from the heat source (hot air) to the heat sink (cold air). The control valves may be manually operated, may be controlled by an automatic operating device such as a thermostat or humidistat, or may be remotely controlled.
In a further embodiment, flow control valves may be installed between the heat pipe""s condenser and the heat pipe""s evaporator to ensure that the interconnecting liquid line(s) do not allow the condensed working fluid to flow from the evaporator back to the condenser. Similarly, in another embodiment, the interconnecting liquid lines may be geometrically configured so that the condensed working fluid is prevented from flowing back to the condenser from the evaporator. Additionally, the interconnecting liquid lines may be configured to prevent vaporized working fluid from flowing out of the condenser to the evaporator until the vaporized fluid is condensed. Further, the interconnecting liquid lines may be configured to prevent vaporized working fluid from flowing out of the evaporator to the condenser through the interconnecting liquid lines. The above indicated mechanisms and configurations function to ensure that vaporized fluid flows from the evaporator to the condenser via the interconnecting vapor line(s), and that condensed fluid flows from the condenser to the evaporator via the interconnecting liquid line(s). When such a flow is maintained, the heat transfer process caused by the heat pipe will be continuous, as long as there is a temperature difference between the heat source (warm air surrounding the heat pipe""s evaporator) and the heat sink (cold air surrounding the heat pipe""s condenser).
Other objects of the present invention are achieved by an HVAC system that utilizes at least one secondary heat transfer unit. In some embodiments, the secondary heat transfer unit is a heat pipe heat exchanger that contains a secondary working fluid. In such an embodiment, the heat pipe contains at least a first section that functions as an evaporator and at least a second section that functions as a condenser. In a further embodiment, the heat pipe may contain at least a third section that also functions as a condenser.
Such an HVAC system further comprises a heat exchanger xe2x80x9cvesselxe2x80x9d that provides a heat source, such as a heated gas or liquid. The vessel may be configured to allow thermal communication between the heat source (contained within the vessel) and the secondary working fluid (contained within the heat pipe). In such a system, the vessel provides heat to the heat pipe""s evaporator. As a result, the working fluid within the evaporator is transformed from a liquid to a vapor. The vaporized working fluid then flows into at least one of the heat pipe""s condensers, where the vaporized fluid thermally communicates with cool air within the HVAC system. The cool air absorbs heat from the vaporized fluid. As such, the cool air is warmed, and the working fluid is condensed. The condensed working fluid flows from the heat pipe""s condenser(s) into the heat pipe""s evaporator, and the heat transfer process continues as long as there is a temperature difference between the heat source (contained within the vessel) and the cool air (within the HVAC system).
Further objects of the present invention are accomplished by an HVAC system that utilizes at least one secondary heat transfer unit. The secondary heat transfer unit has at least a first heat conducting section and a second heat conducting section. The second heat conducting section is at least partially disposed outside the supply duct of the HVAC system.
Preferably, the secondary heat transfer unit employs at least one heat pipe heat exchanger. Further, the heat pipe preferably contains at least one evaporator (a first conducting section) and at least one condenser (a second conducting section). In one such embodiment, the evaporator section is disposed outside the supply duct of the HVAC system, while the condenser section is disposed within the supply duct. Such a configuration provides less resistance to air flowing within the supply duct and allows the evaporator to thermally communicate with air in an outer air passage outside the supply duct.
The heat pipe further contains a secondary working fluid, which is composed of any fluid that readily transfers heat with the environment at the desired temperature conditions. Refrigerants such as freon provide a suitable working fluid. In a heating system embodiment, the working fluid within the heat pipe evaporator thermally communicates with warm air outside the supply duct, so that heat is absorbed by the working fluid. As a result, the working fluid vaporizes within the evaporator. The vaporized working fluid then flows to the heat pipe""s condenser located within the supply duct. Within the condenser, the vaporized fluid releases heat to cool air that is flowing through the supply duct and around the condenser. As a result, the cool air is warmed, and the vaporized fluid is condensed.
The HVAC system may further comprise a blower located outside of the supply duct. The blower functions to control heat transfer between the evaporator section (the first conducting section) and the warm air outside of the supply duct. Additionally, the blower may be controlled by an automated operating device, such as a thermostat or a humidistat. The evaporator section (located outside the supply duct) may be oriented in a variety of configurations. For example, the evaporator section may be parallel with or perpendicular to the condenser section (the second conducting section), which is located within the supply duct. Therefore, the orientation of the evaporator section may be modified according to variables such as space availability outside the supply duct and the direction of air flow caused by the blower in the outer air passage.
In a further embodiment of the present invention, additional control of the HVAC system is provided by at least one other control device located within the HVAC system supply duct. For example, the control device may comprise a motor-actuated damper mechanism having an automated operating device.
Still further objects of the present invention are accomplished by an HVAC system that utilizes at least one secondary heat transfer unit. The secondary heat transfer unit functions to control the transfer of heat within individual heating or cooling zones of a building having a central HVAC system. The secondary heat transfer unit preferably comprises a heat pipe having at least one evaporator section and at least one condenser section. The heat pipe""s evaporator section is disposed in a warm air passage of an individual zone, while the heat pipe""s condenser section is disposed in a cool air passage of another individual zone.
The heat pipe can further contain a secondary working fluid. The working fluid within the evaporator thermally communicates with warm air in the warm air passage. As the working fluid absorbs heat from the warm air, the warm air is cooled, and the working fluid is vaporized. The vaporized working fluid then passes through one or more interconnecting vapor lines (in some embodiments) to the heat pipe""s condenser section. The vaporized fluid within the condenser thermally communicates with cold air in the cold air passage. As the vaporized fluid releases heat into the cold air, the cold air is warmed, and the vaporized fluid condenses. The condensed fluid then returns to the evaporator section through one or more interconnecting liquid lines (in some embodiments).
In a presently preferred embodiment of the present invention, one or more mechanisms may be provided within the heat pipe to precisely control the amount of heat transferred by the heat pipe. These mechanism(s) may interrupt or modulate the flow of the working fluid between the components of the heat pipe. For example, flow control valves may be installed between the evaporator and the condenser to interrupt or reduce the flow of working fluid from the evaporator to the condenser, or vice versa. Such valves can thereby prevent or reduce heat transfer by the heat pipe from the heat source (hot air) to the heat sink (cold air). The control valves may be manually operated, may be controlled by an automatic operating device such as a thermostat or humidistat, or may be remotely controlled.
In a presently preferred embodiment, flow control valves may be installed between the heat pipe""s condenser section and the heat pipe""s evaporator section to ensure that the interconnecting liquid line(s) do not allow the working fluid to flow from the evaporator back to the condenser. Similarly, in another embodiment, the interconnecting liquid lines may be geometrically configured so that the condensed working fluid is prevented from flowing back to the condenser from the evaporator. Additionally, the interconnecting liquid lines may be configured to prevent vaporized working fluid from flowing out of the condenser and traveling to the evaporator until the vaporized fluid is condensed. Further, the interconnecting liquid lines may be configured to prevent vaporized working fluid from flowing out of the evaporator to the condenser through the interconnecting liquid lines. The above indicated mechanisms and configurations function to ensure that vaporized fluid flows from the evaporator to the condenser via the interconnecting vapor line(s), and that condensed fluid flows from the condenser to the evaporator via the interconnecting liquid line(s). When such a flow is maintained, the heat transfer process caused by the heat pipe will be continuous, as long as there is a temperature difference between the heat source (warm air surrounding the heat pipe evaporator) and the heat sink (cold air surrounding the heat pipe condenser).
Other objects, features, and aspects of the present invention are discussed in greater detail below.